Impregnated m lcroporous articles



United States Patent M 3,345,205 IM'PREGNATED MICROPOROUS ARTICLES Harry Raech, .lr., Torrance, Califi, assignor to FMC Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed May 10, 1963, Ser. No. 279,616 6 Claims. (Cl. 117-119) This invention relates to impregnated microporous articles and to an improved method of impregnating microporous articles. More particularly, this invention rel-ates to the impregnation of microporous articles with diallyl phthalate impregnants and to the impregnated articles produced thereby.

In recent years, it has become common practice to impregnate certain microporous articles, particularly metal castings which are intended to retain liquid or gas pressure or vacuum, with a resinous material to reduce their porosity and thereby seal them against leakage. It has been found that microscopic porosities and even porosities which will not pass water, will readily leak lower viscosity liquids such as gasoline and gases. For this reason, even the highest art of casting has not been successful in consistently producing wholly non-porous products, and thus the use of impregnants to overcome microporosity resulting from imperfect castings has become widespread. The practice has been applied to both ferrous and non-ferrous castings including aluminum, magnesium, iron, copper alloys, etc. Metal parts which are commonly impregnated include hydraulic fittings, fuel system fittings, cooling oil fittings, engine blocks, rocket fuel fittings, vacuum and pneumatic fittings, chemical equipment, food equipment, dye castings, forgings, powder metal parts and metal-sprayed parts.

In the evolution of impregnation, natural materials were first used as impregnants, followed by synthetic resins. Among the first ingredients used were tung oil and sodium silicate. However, tung oil is difficult to handle and has only a moderate tank life, while sodium silicate has inadequate permanence, sealing ability and electrical properties. The technically newer and superior systems such as phenolics, alkyds and epoxies have been used with improved success. The most successful of these newer systems is alkyd resins. 1

Although alkyd impregnants give improved performance over previous systems, they also have many problems associated with their use. For example, catalyzed alkyd resin systems have a less desirable storage life, even when kept under refrigeration. Moreover, alkyd impregnants have a tendency to ooze out of the impregnated article during cure. This is due to the fact that alkyd resins lose viscosity when raised to the cure temperature at a faster rate than they gain viscosity.

It is an object of this invention to provide a method of impregnating microporous articles using an impregnant having improved storage life and less tendency to ooze during cure.

Another object is to provide a method of impregnating microporous articles using an impregnant which can be varied over a wide range of viscosities without resulting in any change in the impregnating and curing conditions or the properties of the final impregnated article.

Still another object is to provide impregnated microporous articles having improved solvent and chemical resistance, electrical properties and resistance to leakage at high temperatures.

These and other objects will become apparent from the following description of this invention.

I have now discovered that by impregnating microporous articles with a resinous impreguant which comprises, by weight 60-95 parts of diallyl phthalate monomer, 40 parts of diallyl phthalate prepolymer, and 0.5-5 of a' free-radical initiator, the difficulties encountered in the 3,345,25 Patented Oct. 3, 1967 use of alkyd resin impregnants can be avoided, and the resulting impregnated microporous articles have improved properties, especially superior high temperature leakage resistance.

The impregnants disclosed herein contain a major portion of diallyl phthalate monomer and a minor portion of diallyl phthalate prepolymer. Any of the isomeric forms of diallyl phthalate, that is, ortho-, meta or paraphthalate may be used in accordance with this invention. Diallyl phthalate monomers and prepolymers are wellknown articles of commerce.

Diallyl phthalate prepolymer may be prepared from the monomer by standard polymerization techniques, such as emulsion, solution, or bulk polymerization, usually with a peroxide catalyst, as described by Pollack et al. in Patent No. 2,273,891. A preferred method for conducting this polymerization in a non-aqueous medium using-hydrogen peroxide is described in a copending application of 'C. A. Heiberger, Ser. No. 814,957, filed May 22, 1959.

As a diallyl phthalate polymerization reaction is allowed to proceed, the viscosity of the medium increases slowly at first, and then rapidly as the gel-point of the polymer is approached. The reaction must be terminated before gelation has occurred, normally at about 20-35% conversion of monomer to prepolymer. The polymerization may be terminated by any standard procedure such as lowering the temperature or adding a polymerization inhibitor. The product of this partial polymerization is a dope solution containing prepolymer dissolved in monomer. The prepolymer may be separated from monomer and residual solvent by known procedures, such as by adding a solvent for the monomer which precipitates the partial polymer.

Diallyl phthalate prepolymers are linear or slightly branched, solid polymers containing residual unsaturation. The number average molecular weight of the prepolymer is generally less than about 25,000, and usually about 10,000. These prepolymers are commonly characterized by their precipitated polymer viscosity which is measured as a 25% solution of prepolymer in diallyl phthalate monomer at 25 C. The precipitated polymer viscosity is generally in the range of about -1,000 centipoises, and preferably about 200-700 centipoises.

The optimum composition of the impreguant used in accordance with this invention will vary depending upon the type of article being impregnated and particularly the type of pores involved- When extremely small pores are being impregnated, less viscous solutions containing a higher percentage of monomer are most suitable for complete impregnation. When larger pores are involved, more viscous solutions containing additional prepolymer are most effective, since these solutions contain sufiicient body to prevent withdrawal of the impregnant from the inner pores of the structure, once they have been filled.

The impregnating solutions taughtherein should contain, by weight, 60-95% of diallyl phthalate monomer and 5-40% of diallyl phthalate prepolymer. At least about 5% of prepolymer should be present, thereby providing sufiicient cross-linking sites to avoid the shrinkage and unduly long cure time encountered in the use of larger amounts of monomer. Impregnating solutions containing more than about 40% of prepolymer are too viscous to achieve good penetration of the pores of the article. In general, the viscosity of the impregnating solution should be in the range of about 20-6000 centipoises at 25 C. In most cases, it is preferred that the impregnating solu tion'contain about 70-90 parts of diallyl phthalate monomer for each 10-30 parts of prepolymer. These solutions have viscosities of about 30-800 centipoises.

' The dope solution obtained by the partial polymerization of diallyl phthalate may be used directly as the V impregnating solution disclosed herein, or it may be altered to any desired composition by the addition of more monomer or prepolymer. Of course, the impregnant can also be prepared by dissolving the desired amount of solid prepolymer in monomer.

The impregnating solution should also contain a catalyst to cure the resinous impregnant. Any of the freeradical initiators conventionally used to cure diallyl phthalate prepolymers may be employed. Suitable catalysts include peroxides such as benzoyl peroxide, para-chlorobenzoyl peroxide, 2,4-dichlorbenzoyl peroxide, lauroyl peroxide, di(tert-butyl) peroxide, dicumyl peroxide, cyclohexane peroxide, methyl ethyl ketone peroxide and succinic acid peroxide; hydroperoxides such as tert-butyl hydroperoxide, cumene hydroperoxide, para-menthane hydroperoxide, pinane hydroperoxide, and 2,5-dimethylhexane-2,S-dihydroperoxide; peroxy esters such as tertbutyl perbenzoate, di(tert-butyl) diperphthalate, tertbutyl peracetate, isopropyl percarbonate and 2,5-dimethylhexane-2,5-diperbenzoate; and azo compounds such as azobisiso'buty-ronitrile.

The impregnating solution generally contains about 0.2-5 of catalyst. When less than about 0.2% of catalyst is present, an insufiicient degree of cure may result. More than about 5% of catalyst can be employed, if desired; however, such excesses have a tendency to soften the product. Preferably, about 13% of an organic peroxide is employed.

The catalyzed impregnating solutions taught herein have outstanding stability properties. Unlike typical alkyd resin impregnants of the prior art, diallyl phthalate impregnants do not have to be refrigerated during storage. At room temperature they have a storage life in excess of about 6 months; and thus, with constant replacement of new impregnant in the treating tank, the storage life, for all practical purposes, is infinite.

The microporous article impregnated in accordance with this invention may be composed of any material having micropores. Typical materials include metals such as iron, steel, aluminum, magnesium, and alloys containing these metals as well as copper alloys, and other microporous materials such as ceramics and fibrous materials such as asbestos.

As previously pointed out, the impregnating method of this invention is particularly useful in sealing metal castings against leakage. It is also useful in sealing castings which require subsequent finishing, such as plating or painting. Before finishing, these castings become filled with moisture by entrainment of cleaning solutions and condensation of water vapor from the atmosphere. As a result, when the casting is plated or painted, there is a tendency toward internal corrosion and lifting of the film, such as has been widely experienced with certain automotive par'ts. These defects can be prevented by impregnation of the microporous article with the diallyl phthalate impregnant taught herein prior to plating or painting of the article, resulting in high quality products with excellent life expectancy. I have found that even powder metal parts, which are inherently quite porous, can be successfully plated after impregnation with the impregating solution taught herein.

Impregnation in accordance with this invention also prevents the possibility of galvanic corrosion in metal parts, such as occurs when electrolytes, such as sea water, acidic liquids, etc. penetrate into the pores of metals, and more particularly into crevices between dissimilar metals as in the case of metal inserts and weldments. The liquid within the pores and crevices develop concentration cells which cause electrolytic consumption of the metal. By impreg- The impregnation of microporous articles in accordance with this invention is carried out in essentially the same manner as has been previously employed, except that a different impregnating solution is used. The microporous article is placed in a pressure chamber such as an autoclave and a vacuum of at least about 26 inches of mercury is applied for sufficient time to substantially completely evacuate the pores of the article. Preferably, a vacuum of about 27 inches of mercury or more is employed. The actual time required to evacuate the pores of the article will vary depending upon the amount of vacuum and the type of pores involved. For example, a vacuum of about 27 inches of mercury should be applied for about 30 minutes to completely evacuate the pores of a typical metal casting. In general, the length of time required for evacuation will be about 20-60 minutes.

After the pores have been completely evacuated, the impregnating solution is allowed to slowly enter the vacuum chamber, while maintaining the vacuum. When the porous article is completely submerged in the impregnant, the vacuum is released and a pressure of at least about 50 psi. is applied thereby driving the impregnant into the pores of the article. The length of time that pressure is applied will depend upon the type of pores and the amount of pressure used. For example, at a pressure of psi, about 30 minutes are required to completely impregnate the pores of a typical metal casting.

After the porous article has been completely impregnated, the pressure is released, and the porous article is removed from the body of impregnant and drained of excess impregnant. When using the impregnating solution of this invention, impregnant still adhering to the surface of the article is readily removed by rinsing the impregnated article at ambient temperature in an aqueous detergent solution or with diallyl phthalate monomer. One of the advantages of using the impregnants taught herein is that the flammable organic solvents required to clean articles impregnated with typical alkyd resin impregnants of the prior art are not necessary.

After the impregnated article has been cleaned, it is cured in an oven at a temperature in the range of about 20-350 F. The cure time may vary from about 30-90 minutes, depending upon the temperature used and the size of impregnated article. In contrast to typical alkyd resin impregnants of the prior art, the impregnating solutions used in accordance with this invention cure satisfactorily over a wide range of temperatures and result in no exudation or bleeding of resin from the impregnated article during cure. When using typical prior art alkyd resin impregnants, it is necessary to remove the exudate by an additional cleaning step at this point. This cleaning step can be omitted when operating in accordance with this invention.

Another very important advantage of the impregnating solutions taught herein is that, by changing the ratio of monomer to partial polymer, a solution having the optimum viscosity for proper impregnation of any particular microporous article can be employed without altering the impregnating and curing conditions or the properties of the final impregnated article. This is possible because diallyl phthalate and its partial polymer form the same resin structure when cured. The cured impregnants disclosed herein develop outstanding physical properties, particularly solvent and chemical resistance,

, electrical properties and resistance to leakage at elevated Eight hundred eighty six parts of diallyl ortho-phthalate monomer, 62.2 parts of isopropanol (91% by vol.) and 7.5 parts of hydrogen peroxide (50.5% H 0 were charged to a stainless steel reactor, thoroughly agitated and heated to a pot temperature of 104108 C. at total reflux. After hrs. the viscosity of the reaction mixture had increased to 27 cps. at 106 C., as measured by a Bendix Ultraviscotron viscometer. The batch was cooled to obtain a reaction product mixture containing 27% prepolymer, 67% unreacted monomer and 6% isopropanol and having a viscosity of 425 cps. at 25 C. This reaction product mixture was distilled to remove the isopropanol yielding a dope solution containing 28.7% prepolymer dissolved in monomer.

An impregnating solution was prepared as follows: Organic peroxide and additional diallyl phthalate mono mer Were added to a portion of the dope solution prepared above to provide an impregnating solution having the following composition Ingredient: Parts Diallyl ortho phthalate monomer 80 Diallyl ortho-phthalate prepolymer Tert-b utyl perbenzoate 1.5

This impregnating solution had a viscosity of 135 cps., a specific gravity of 1.14, both at 23 C., and an SPI gel time of 12.5 minutes at a temperature of 250 F.

Aluminum test cups were pretested for leakage as follows: Three clean, dry aluminum test cups, having an outside diameter of 3% in. at the top, 2% in. at the bottom, and a wall thickness of A in., were sealed by clamping a lid and gasket to the top of each cup and connecting the lid to an air source. Each cup was pretested for leakage by placing the assembly in a test chamber equipped with a Precision Wet Test Meter and subjecting it to an internal aerostatic pressure of 25 p.s.i. of dry air. The cups had the following leakages.

These test cups were impregnated as follows: Each of the aluminum cups was placed in an impregnating vessel and a vacuum of 27.5 in. Hg was applied for a period of min. The impregnating solution was then allowed to slowly enter the impregnating chamber, while maintaining the vacuum until the cup was completely submerged in the impregnating solution at room temperature. The vacuum was then released, and 100 p.s.i. air pressure was applied to the impregnating vessel and held for 30 min. The pressure was then released and the cup was removed from the impregnating medium, drained of excess impregnant, rinsed in a tripolyphosphate detergent solution, placed in a circulating air oven, and cured at 300 F. for 1 hr. After curing, the cup was removed from the oven and allowed to cool to room temperature.

The impregnated cups were tested for leakage as (follows: A lid and a rubber gasket was clamped to the top of each of the cups, the lids were connected to a high pressure air source, and the assembly was immersed in water. An internal aerostatic pressure of 80 p.s.i. was applied. No leakage of air was observed from any of the cups.

Example 2 Diallyl isophthalate was polymerized as follows: One hundred parts of diallyl isophthalate monomer was mixed with 0.15 part of tert-butyl hydroperoxide and 0.086 part of hydrogen peroxide and heated with stirring to 120 C. After 7.5 hrs. at that temperature, the reaction was terminated and the mass contained 22% prepolymer and had a viscosity of 350 cps. at 25 C.

By adding catalyst and more diallyl isophthalate to this dope solution, an impregnating solution was prepared having the following composition Ingredient: Parts Diallyl isophthalate monomer Diallyl isophthalate prepolymer 15 Tert-butyl hydroperoxide 2 Three aluminum test cups having leakages of 0.38, 1.45 and 2.4 were impregnated with the above solution using the procedure of Example 1 and cured at a temperature of 275 F. for 1.5 hrs. The cups were tested as in Example 1 and found to have no leakage.

Example 3 Diallyl phthalate prepolymer was prepared as follows:

Precipitated polymer viscosity (25% solution in monomer at 25 C.) cps. 354 Softening range, C. 80405 Iodine number 55 Specific gravity at 25 C. AS'DM D792-50) 1.267

An impregnating solution was prepared as follows: A portion of the above diallyl phthalate prepolymer and catalyst were added to the dope solution prepared in Example 1 to give an impregnating solution having the following composition Ingredient: Parts Diallyl ortho-pbthalate monomer 65 Diallyl ortho-phthalate prepolymer 35 Dicumyl peroxide 1.5

A ferrous-alloy powder metal part was impregnated as indicated in Example 1 using the above solution. The impregnated part was cured at 300 F. for 1 hr., cooled to room temperature and found to be leakproof.

Example 4 A copper-alloy hydraulic valve body casting was impregnated as indicated in Example 1 with a solution having the composition-- Ingredient: Parts Diallyl ortho-phthalate monomer Diallyl ortho-phthalate prepolymer 10 Benzoyl peroxide 2 The impregnated valve body was cured at 340 F. for 1 hr. The part did not leak in use.

Example 5 A ceramic insulator was impregnated as indicated in Example 1 with a solution having the following composition- Ingredient: Parts Diallyl ortho-phthalate monomer 75 Diallyl ortho-phthalate prepolymer 25 Tert-butyl perbenzoate 1.5

The impregnated insulator was cured at 300 F. for 1 hr. This insulator maintained its electrical properties under high humidity and high electrical stress.

Example 6 A fiberinsulator part was impregnated as indicated in Example 1 with a solution having the composition Ingredient: Parts Diallyl ortho-phthalate monomer 75 Diallyl ortho-phthalate prepolymer 25 Tert-butyl perbenzoate 1.5

The impregnated part was cured at 300 F. for 1 hr. This insulator maintained good electrical properties during use and did not support the growth of fungi.

As will be apparent to those skilled in the art, numerous modifications and variations of the embodiments illustrated above may be made without departing from the spirit of the invention or the scope of the following claims.

I claim:

1. In the method of impregnating metal and ceramic microporous articles by evacuating the pores of the article, filling the pores of the article with a catalyzed resinous impregnant and curing the resinous impregnant by heat, the improvement which comprises using a resinous imprcgnant consisting essentially of, by weight, 60-95 parts of diallyl phthalate monomer, -40 parts of diallyl phthalate prepolymer and 0.2-5% of a free-radical initiator.

2. The method of claim 1, in which the resinous impregnant contains, by weight, 70-90 parts of diallyl phthalate monomer, 10-30 parts of diallyl phthalate prepolymer and 1-3% of an organic peroxide.

3. The method of claim 2, in which the microporous article is a metal casting.

4. An impregnated microporous article having improved properties in which the pores are impregnated with a cured diallyl phthalate resin derived from an impregnating solution containing, by weight, 60-95 parts of diallyl phthalate monomer, 5-40 parts of diallyl phthalate prepolymer, and 0.2-5% of a free-radical initiator.

References Cited UNITED STATES PATENTS 8/1948 Wagers et al. 10/1961 Hough et al.

OTHER REFERENCES Burnett: Allyl Resins and Monomers, Modern Plastic Encyclopedia 62, vol. 39, No. 1A, September 1961, pp. 153-156, TP986.A2M5.

Knight: Metals and Alloys, vol. 20, December 1944, pp. 1625-1630.

Plastics Uses of Diallyl Phthalate, The Chemical Age, February 11, 1960, pp. 226, 228, TP 1 (1.36.

ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,345,205 October 3, 1967 Harry Raech, Jr.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 42, for "20" read 200 Signed and sealed this 14th day of January 1969.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. IN THE METHOD OF IMPREGNATING METAL AND CERAMIC MICROPOROUS ARTICLES BY EVACUATING THE PORES OF THE ARTICLE, FILLING THE PORES OF THE ARTICLE WITH A CATALYZED RESINOUS IMPREGANT AND CURING THE RESINOUS IMPREGNANT BY HEAT, THE IMPROVEMENT WHICH COMPRISES USING A RESINOUS IMPREGNANT CONSISTING ESSENTIALLY OF, BY WEIGHT, 60-95 PARTS OF DIALLYL PHTHALATE MONOMER, 5-40 PARTS OF DIALLYL PHTHALATE PREPOLYMER AND 0.2-5% OF A FRER-RADICAL INITIATOR. 